1. Field of the Invention
The present invention relates to a photopolymerizable composition and more specifically to a photopolymerizable composition which has high sensitivity and can be used for preparing a lithographic printing plate, a resin-letterpress plate, a resist or photomask for making a printed board, a monochromatic and colored sheet for transfer or color-development and a color-developing sheet.
2. Prior Art
There have conventionally been known a variety of photographic image-forming systems in which a ardenable organic component carrying an ethylenically unsaturated group is used for forming images. The organic component can undergoes an optically induced addition reaction of the ethylenically unsaturated groups, typically a polymerization or crosslinking reaction to cause hardening to thus give images.
As other photographic image-forming systems, there have also been known systems which comprise a combination of a leuco dye which can develop the color through an oxidation reaction or a reaction with an acid and a radical-generating agent which acts as an oxidizing agent or an acid. Examples of such systems are disclosed in, for instance, Phot. Sci. Eng., p. 598 (1961); and U.S. Pat. Nos. 3,042,515 and 3,615,568.
Among these image-forming methods which comprise the use of a light-sensitive composition containing a hardenable organic component carrying an ethylenically unsaturated group, presently there has widely been well known a method which comprises duplicating images through a photographic technique using a light-sensitive composition comprising, for instance, a polymerizable ethylenically unsaturated compound, a photopolymerization initiator, a linear polymeric compound having a film-forming ability and a heat polymerization inhibitor. As described in, for instance, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") Nos. Sho 35-5093 and Sho 35-8495, the light-sensitive composition causes photopolymerization by irradiation of actinic light rays and is correspondingly made insoluble. Therefore, a desired photopolymerized images can be obtained by applying a light-sensitive composition in the form of a proper film, irradiating the light-sensitive layer with actinic light rays through a negative film carrying a desired image and then removing only the unexposed areas with a proper solvent (hereinafter referred to as simply "development"). The light-sensitive composition of this type is very useful as the light-sensitive layer of a presensitized plate for use in making a lithographic printing plate (hereinafter referred to as "PS plate"). Typical examples of photopolymerization initiators for these photopolymerizable ethylenically unsaturated compounds include benzyl, benzoin, benzoin ethyl ether, anthraquinone, acridine, phenazine, benzophenone and 2-ethylanthraquinone. However, these photopolymerization initiators are responsive o ultraviolet rays having short wavelengths, but have low sensitivity and further is not almost responsive to visible light rays such as argon ion laser rays having long wavelengths and thus have insufficient ability of initiating polymerization.
On the other hand, there has been forwarded the development of light-sensitive materials responsive to visible light rays and there have been proposed, for instance, a composite system comprising an photoreducible dye and an aliphatic amine as disclosed in J.P. KOKOKU No. Sho 44-20189; a composite system comprising a cyclic cis-.alpha.-dicarbonyl compound and a dye as disclosed in Japanese Patent Unexamined Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 48-84183; a system comprising a hexaaryl biimidazole, a radical generator and a dye as disclosed in J.P. KOKOKU No. Sho 45-37377; a system comprising a hexaaryl biimidazole and a p-dialkylaminobenzylideneacetone as disclosed in J.P. KOKAI Nos. Sho 47-2528 and Sho 54-155292; and a system comprising a substituted triazine and a merocyanine dye as disclosed in J.P. KOKAI No. Sho 54-151024.
Recently, there has been investigated a method for forming images by using a laser and a laser facsimile or the like has already been put to practical use. For this reason, there has correspondingly been desired for the development of light-sensitive composition having high sensitivity applicable to these recently-developed methods, but any composition having sufficient sensitivity has not yet been developed.